Thermoplastic olefins (TPOs), impact copolymers (ICPs) and thermoplastic vulcanizates (TPVs), collectively referred to herein as heterophase polymer compositions, thermoplastic elastomers and/or soft thermoplastic blends (STPBs), comprise an isotactic polypropylene thermoplastic phase and a high molecular weight or crosslinked elastomeric phase. The STPBs also commonly includes non-polymeric components such as fillers and other compounding ingredients.
STPBs are multiphase polymer blends where a thermoplastic such as isotactic polypropylene forms a continuous matrix phase and an elastomer, such as an ethylene containing interpolymer, is the dispersed phase. The polypropylene matrix imparts tensile strength and chemical resistance to the STPBs, while the elastomer imparts flexibility and impact resistance. A distinction between these members of STPBs, (e.g. TPVs, TPOs and ICPs) is some have a dispersed phase which is not crosslinked, or is only modestly crosslinked, others have crosslinked dispersed phases. TPOs are made by mechanical and post polymerization blending of the components while ICPs are made during the polymerization by differential polymerization of the polymer components. TPVs are also blends of thermoplastic and elastomer like TPOs, except that the dispersed elastomer component is crosslinked or vulcanized. As a result of the increased coherence of the dispersed, crosslinked elastomer, its share of the total composition in a TPV may be increased to levels higher than for a TPO.
Traditionally, highly amorphous, very low density ethylene-propylene copolymers (EPs) and ethylene-propylene-diene terpolymers (EPDMs) have been used as the elastomer component in STPBs. These EPs or EPDMs generally have a high viscosity expressed in Mooney units. STPBs desirably have the processing characteristics of the thermoplastic phase, while the ultimate properties include a substantial amount of elasticity from the rubber phase.
A major market for STPBs is in the manufacture of automotive parts, especially bumper fascia, door skin, air bag cover, side pillars and the like. These parts are generally made using an injection molding process. To increase efficiency and reduce costs it is necessary to decrease molding times and reduce wall thickness in the molds. To accomplish these goals, manufacturers have turned to high melt flow rate polypropylenes, (e.g. greater than 35 g/10 min). These high melt flow rate resins are low in molecular weight and consequently difficult to toughen, resulting in products that have low impact strength as previously mentioned. It would be desirable to formulate an STPB having greater elongation to break and more toughness, improved processability, and/or a combination thereof.
US 2007/0240605 describes low molecular weight propylene copolymers. Polyolefin thermoplastic elastomers comprising blends of propylene polymers with non-crystalline ethylene-α-olefin random copolymers or with hydrogenated products of styrene-butadiene-styrene block copolymers are disclosed in Japanese laid-open patent application Nos. Sho 50-14742/1975, Sho 52-65551/1977, Sho 58-20664/1983, and Sho 58-215446/1983.